Recently, energy saving for an electrophotographic image forming apparatus has been desired in view of countermeasure of global environment contamination, and, specifically, energy saving for a fixing system which consumes a big amount of energy among an electrophotographic image forming apparatus has been desired.
So far, a resin having sharp melt property, specifically, a crystalline polyester resin, has been known to be used in a toner as a binder resin for one of effective methods for low temperature fixing. Further, a method to use a polyester resin having a crosslinking structure having excellent high temperature elasticity has been known to dissolve problems such as poor anti-high temperature offset property, poor high temperature storage property or too much gloss caused by using a crystalline polyester resin (see, for example, patent document 1).
However, it is difficult to produce a toner containing a polyester resin having a crosslinking structure and having a small particle diameter by the above described toner producing method, according to the following reason, and, therefore, an image having high quality is difficult to be formed by using such a toner.
Namely, it has been known that a toner having a small particle diameter can be produced by employing a polymerization method. However, it is difficult to produce a toner containing a polyester resin since it is difficult to fully conduct the reaction in an aqueous medium because polyester is obtained by a dehydration condensation reaction. As a countermeasure, there has been proposed a method in which a polyester resin preliminarily formed by a dehydration condensation reaction is dissolved in an organic solvent, and dispersed in an aqueous medium, for example, by a phase-transfer emulsification method, to obtain an emulsion liquid, whereby toner particles are formed using the emulsion liquid. However, even when such a method is used, there is a problem because the polyester resin having a crosslinking structure is difficult to be dissolved or dispersed in an organic solvent and needs plenty of energy to dissolve or disperse the polyester resin.
In order to solve the above mentioned problem, employed is a method to use a polyester in which an isocyanate group is introduced to form a crosslinking structure simultaneously when the particles are formed (see, for example, patent documents 2 and 3).
However, the problem is that it is difficult to control the reaction and impossible to stably produce a toner because of the extremely high reactivity of the isocyanate group.
In order to overcome such a problem, there has been proposed a method to introduce a polyester resin having a crosslinking structure into toner particles by coating the circumference of core particles via adhesion of polyester resin particles having a crosslinking structure to the core particles (for example, refer to patent document 4).
However, the aforementioned method disclosed by patent document 4 has a problem that the capable range of fixing temperature is narrow due to incorporation of toner particles having a lowered temperature causing high-temperature offset. This is because, when the polyester particles having a crosslinking structure are formed via a phase-transfer emulsification method, a crosslinking reaction has been already started at the moment when the oil phase is dispersed in an aqueous medium, because the oil phase liquid obtained by dissolving a polyester resin containing a polymerizable unsaturated bond in an organic solvent already contains a radical polymerization initiator, whereby the variation in the size of oil droplets formed from the oil phase or the variation in the crosslinking degree in each oil droplet becomes larger.